Front/Back Focus issue

Lev · 01-25-2018, 11:05 AM

Today I was at local shop, checking lens from top to bottom, and we discovered that the DOF is moving front back by at least -2 +2 almost every time you take picture at 50mm 2.8, 1m distance. Didn't check this at 70mm but I'm sure that it will be less. This guy told me that there were tons of lenses with similar effect and suggested to keep this lens. :/

Totally confused

**BarryE** · 01-25-2018, 12:44 PM

At effective FL of 75mm and at f2.8 the accepted range in focus at 1m will be narrow. I assume you were using spot focus ? If you take the same shot, refocusing every time, there will be a distribution in the point of focus the AF system measures. Add into the mix the absolute need for the camera and the subject to not move at all, then it hardly surprises me, that in a shop environment, you saw differences.

When I set my AF settings I always take at least 3, if not 5 shots and with very careful securing of camera and subject, MUP, remote etc, I will expect to get a distribution. Even a change in light will throw things as the subject will 'look' different to the camera. It's a problem in sunny/cloudy days and inside with flourescent lights too.

I wish focusing was objective, but in reality an AF system gets you close, but when the margins are small, a combination of manual/AF/bracketing/multiple shots etc will be the only way to get some shots at large apertures.

Lev · 01-25-2018, 01:20 PM

Originally posted by BarryE

I assume you were using spot focus ?

Yes

Originally posted by BarryE

When I set my AF settings I always take at least 3, if not 5 shots

True if you or your subject is moving and your camera follows your focus point, but in my case, every shot will be front/back focused if I refocus. How much this focus throw is acceptable? how much it should be plus or minus? Again, I've compared this on my 16-85, I know it's 4.0 against 2.8 but there's just tiny shift, I've done 10 shots with 16-85 10 out of 10 its focus point was sharp, slight deviation of DOF, I mean, really slight shift, but every time it focused well. With 50-135 I have maybe 3 shots with sharp focus point. I'm not saying that it must be same every time, but the difference is huge!

You're right about conditions but we made this test in constant light, same environment for both lenses. Sadly, every time I wish to somehow keep it - new thing is coming.

pschlute · 01-25-2018, 06:06 PM

Originally posted by Lev

With 50-135 I have maybe 3 shots with sharp focus point. I'm not saying that it must be same every time, but the difference is huge!

You're right about conditions but we made this test in constant light, same environment for both lenses

if you get a high percentage of correctly auto focused results from your other lenses but only 3/10 with this lens then i would send it back for a replacement

Lev · 01-26-2018, 08:25 AM

In addition to all this stuff now focusing ring started to squeak.

I don't understand how all these things can happen in a brand new product? I'm just very much wondering where the hell QC department was, on vacation? The only properly working part is glass, absolutely stunning, which I'm not sure I'll get in another copy.

Now I'm definitely going for another risk by exchanging this lens. Unfortunately I have no other options. If I'll get another bad copy I'm done.

Lev · 01-29-2018, 02:34 AM

sent it back

**BarryE** · 01-29-2018, 02:58 AM

Originally posted by Lev

sent it back

Hope the replacement works better for you. It is an excellent lens (or should be).

Lev · 01-29-2018, 03:12 AM

Originally posted by BarryE

Hope the replacement works better for you. It is an excellent lens (or should be).

Thanks but I'm thinking maybe 70-200 would be better in terms of quality control? I've already payed 1/3 of this price of this lens and have no idea what I will get in replacement

I'm getting sick, I can't remember last time I bought something from B&H and it was good and didn't need to be returned :/

Lev · 01-29-2018, 12:02 PM

Guys,

I've made some shots before sending it in. Just take a look how DOF is moving between shots, all using center point, re-focusing everytime:

108mm, 2.8, AF Fine Tune: 0

I was aiming at the center of white circle. Backfocusing.

Right after:

50mm, 2.8, AF Fine Tune: 0

I was aiming at the keys. Frontfocusing.

These are shots when I've realized that the ftront/backfocusing appear to be somehow not stable and moving shot to shot, AF fine tune setting was set to +4. Now target is third chain from left:

75mm, 2.8; AF Fine Tune +4

I don't know how you guys see differences but at high resolution I see only one image with tack sharp chain out of 10!

how do you think

pschlute · 01-29-2018, 01:37 PM

If you want to test front/back focus you need to do it scientifically with a test chart. Not handheld, and not by using a variety of different images.

Test the new lens properly when it arrives.

**BarryE** · 01-29-2018, 02:03 PM

Originally posted by pschlute

If you want to test front/back focus you need to do it scientifically with a test chart. Not handheld, and not by using a variety of different images.

Test the new lens properly when it arrives.

I think this has been said.

There are too many variables without making accurate tests, not least the inherent distribution of focus points even when every thing is done just right.

Lev · 01-29-2018, 02:56 PM

What's up to you guys? We've done that test charts at shop. Autofocus was off at 50mm and 90mm. I've just uploaded these images as examples. I didn't want you to tell me whether it is off indeed. I was asking what do you think about it. The guy at shop told me that this is normal, he show me ultra fast Canon glass with same thing. Ok normal, but I was ending up 80% with blurry images.

pschlute · 01-29-2018, 04:27 PM

Originally posted by Lev

What's up to you guys? We've done that test charts at shop. Autofocus was off at 50mm and 90mm. I've just uploaded these images as examples. I didn't want you to tell me whether it is off indeed. I was asking what do you think about it. The guy at shop told me that this is normal, he show me ultra fast Canon glass with same thing. Ok normal, but I was ending up 80% with blurry images.

I have told you what I think. If you get only 20% correctly focused shots using a proper AF testing technique , there is something wrong with the lens or the camera.

If the camera shop tell that is normal they are lying to you.

Lev · 01-30-2018, 02:08 AM

ok pschlute, I just thought maybe it was related to the loose focusing ring I had with this lens? Some folks here confirmed that they also had this little backlash in their copies, I just wonder how is their focusing accuracy?

01-25-2018, 01:20 PM	#18
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	Originally posted by BarryE I assume you were using spot focus ? Yes Originally posted by BarryE When I set my AF settings I always take at least 3, if not 5 shots True if you or your subject is moving and your camera follows your focus point, but in my case, every shot will be front/back focused if I refocus. How much this focus throw is acceptable? how much it should be plus or minus? Again, I've compared this on my 16-85, I know it's 4.0 against 2.8 but there's just tiny shift, I've done 10 shots with 16-85 10 out of 10 its focus point was sharp, slight deviation of DOF, I mean, really slight shift, but every time it focused well. With 50-135 I have maybe 3 shots with sharp focus point. I'm not saying that it must be same every time, but the difference is huge! You're right about conditions but we made this test in constant light, same environment for both lenses. Sadly, every time I wish to somehow keep it - new thing is coming. Last edited by Lev; 01-25-2018 at 02:56 PM.

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01-25-2018, 11:05 AM	#16
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	Today I was at local shop, checking lens from top to bottom, and we discovered that the DOF is moving front back by at least -2 +2 almost every time you take picture at 50mm 2.8, 1m distance. Didn't check this at 70mm but I'm sure that it will be less. This guy told me that there were tons of lenses with similar effect and suggested to keep this lens. :/ Totally confused

01-25-2018, 12:44 PM	#17
BarryE Site Supporter Join Date: Jan 2013 Location: Hampshire, UK Posts: 1,653	At effective FL of 75mm and at f2.8 the accepted range in focus at 1m will be narrow. I assume you were using spot focus ? If you take the same shot, refocusing every time, there will be a distribution in the point of focus the AF system measures. Add into the mix the absolute need for the camera and the subject to not move at all, then it hardly surprises me, that in a shop environment, you saw differences. When I set my AF settings I always take at least 3, if not 5 shots and with very careful securing of camera and subject, MUP, remote etc, I will expect to get a distribution. Even a change in light will throw things as the subject will 'look' different to the camera. It's a problem in sunny/cloudy days and inside with flourescent lights too. I wish focusing was objective, but in reality an AF system gets you close, but when the margins are small, a combination of manual/AF/bracketing/multiple shots etc will be the only way to get some shots at large apertures.

01-25-2018, 06:06 PM	#19
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,193	Originally posted by Lev With 50-135 I have maybe 3 shots with sharp focus point. I'm not saying that it must be same every time, but the difference is huge! You're right about conditions but we made this test in constant light, same environment for both lenses if you get a high percentage of correctly auto focused results from your other lenses but only 3/10 with this lens then i would send it back for a replacement

01-26-2018, 08:25 AM	#20
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	In addition to all this stuff now focusing ring started to squeak. I don't understand how all these things can happen in a brand new product? I'm just very much wondering where the hell QC department was, on vacation? The only properly working part is glass, absolutely stunning, which I'm not sure I'll get in another copy. Now I'm definitely going for another risk by exchanging this lens. Unfortunately I have no other options. If I'll get another bad copy I'm done.

01-29-2018, 02:34 AM	#21
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	sent it back

01-29-2018, 02:58 AM	#22
BarryE Site Supporter Join Date: Jan 2013 Location: Hampshire, UK Posts: 1,653	Originally posted by Lev sent it back Hope the replacement works better for you. It is an excellent lens (or should be).

01-29-2018, 03:12 AM	#23
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	Originally posted by BarryE Hope the replacement works better for you. It is an excellent lens (or should be). Thanks but I'm thinking maybe 70-200 would be better in terms of quality control? I've already payed 1/3 of this price of this lens and have no idea what I will get in replacement I'm getting sick, I can't remember last time I bought something from B&H and it was good and didn't need to be returned :/

01-29-2018, 12:02 PM	#24
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	Guys, I've made some shots before sending it in. Just take a look how DOF is moving between shots, all using center point, re-focusing everytime: 108mm, 2.8, AF Fine Tune: 0 I was aiming at the center of white circle. Backfocusing. Right after: 50mm, 2.8, AF Fine Tune: 0 I was aiming at the keys. Frontfocusing. These are shots when I've realized that the ftront/backfocusing appear to be somehow not stable and moving shot to shot, AF fine tune setting was set to +4. Now target is third chain from left: 75mm, 2.8; AF Fine Tune +4 I don't know how you guys see differences but at high resolution I see only one image with tack sharp chain out of 10! how do you think

01-29-2018, 01:37 PM	#25
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,193	If you want to test front/back focus you need to do it scientifically with a test chart. Not handheld, and not by using a variety of different images. Test the new lens properly when it arrives.

01-29-2018, 02:03 PM	#26
BarryE Site Supporter Join Date: Jan 2013 Location: Hampshire, UK Posts: 1,653	Originally posted by pschlute If you want to test front/back focus you need to do it scientifically with a test chart. Not handheld, and not by using a variety of different images. Test the new lens properly when it arrives. I think this has been said. There are too many variables without making accurate tests, not least the inherent distribution of focus points even when every thing is done just right.

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01-29-2018, 02:56 PM	#27
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	What's up to you guys? We've done that test charts at shop. Autofocus was off at 50mm and 90mm. I've just uploaded these images as examples. I didn't want you to tell me whether it is off indeed. I was asking what do you think about it. The guy at shop told me that this is normal, he show me ultra fast Canon glass with same thing. Ok normal, but I was ending up 80% with blurry images.

01-29-2018, 04:27 PM	#28
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,193	Originally posted by Lev What's up to you guys? We've done that test charts at shop. Autofocus was off at 50mm and 90mm. I've just uploaded these images as examples. I didn't want you to tell me whether it is off indeed. I was asking what do you think about it. The guy at shop told me that this is normal, he show me ultra fast Canon glass with same thing. Ok normal, but I was ending up 80% with blurry images. I have told you what I think. If you get only 20% correctly focused shots using a proper AF testing technique , there is something wrong with the lens or the camera. If the camera shop tell that is normal they are lying to you.

01-30-2018, 02:08 AM	#29
Lev Veteran Member Join Date: Aug 2015 Location: Tbilisi, Georgia Posts: 1,197 Original Poster	ok pschlute, I just thought maybe it was related to the loose focusing ring I had with this lens? Some folks here confirmed that they also had this little backlash in their copies, I just wonder how is their focusing accuracy?